This invention relates to combined-cycle thermal energy and power systems and, specifically, to systems which combine gas turbines, steam generators, heat recovery steam generators, generators and associated controls for the production of electrical power.
Currently available combined-cycle systems of the assignee of this invention include single and multi-shaft configurations. Single shaft configurations consist of one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG). The gas turbine and steam turbine are coupled to the single generator in a tandem arrangement on a single shaft. Multi-shaft systems, on the other hand, have one or more gas turbine-generators and HRSG's that supply steam through a common steam header to a single steam turbine-generator. In either case, steam is generated in one or more unfired HRSG's for delivery to the condensing steam turbine.
Conventional practice for reheat steam cycles utilizing reheat is generally to configure the steam system with one steam turbine to one HRSG. In a single gas turbine/steam turbine/HRSG system, condensate from the condenser is pumped directly to the HRSG where steam is generated by heat supplied by the gas turbine exhaust gases, and is thereafter returned to the steam turbine.
In the conventional practice, the temperature of steam admitted to the steam turbine throttle, i.e., the main steam, and the temperature of the reheated steam are set at the maximum permitted by the materials employed in the boiler superheaters, steam piping, valves and steam turbines. Exemplary temperatures are in the range of 1000.degree. F. (538.degree. C.) to 1050.degree. F. (566.degree. C.). It is also conventional practice to locate the high temperature section of the HRSG reheater in that section of the HRSG where gas turbine exhaust gas temperature is highest, i.e., adjacent the gas turbine exhaust gas inlet to the HRSG. While this arrangement is satisfactory in single gas turbine/steam turbine/HRSG systems, problems arise in multiple gas turbine/HRSG, single steam turbine systems which would otherwise be preferred.
In reheat combined cycle systems where there are multiple gas turbines and multiple HRSG's for a single steam turbine, the steam from the superheater in each HRSG is combined and flows to the high pressure (HP) section of the steam turbine. The steam passes through the HP turbine where it is extracted for reheating. The steam exhausting from the HP turbine must be distributed uniformly to the reheaters in the multiple HRSG's since reduced flow to one reheater will result in higher operating temperature for that reheater, potentially damaging it if the temperature is increased above its operating limit as a result of heat damage to the reheater tube material. This process is inherently unstable since increased temperature of the steam passing through the reheater with lowest flow increases the specific volume and velocity of the steam, which further increases its pressure drop, thus further reducing the flow which causes the tube wall temperature to increase even further. Therefore, a control system is required consisting of main steam flow measurement, cold reheat steam flow control valves, and a master control system to match the main steam flow and reheat steam flow for each HRSG.
A second problem that requires solution for a successful combined cycle system as described above is prevention of overheating of the reheater during system start-up. A system where the high temperature sections of the reheaters are exposed to the highest exhaust gas temperatures requires some means for cooling the reheater during start-up of the HRSG, prior to admitting steam to the steam turbine, i.e., when there is no steam flow from the HP section of the steam turbine to the reheater. The reheater can be cooled as done in conventional steam turbine practice by temporarily connecting (during starting) the steam from the HRSG superheater to the reheater, then to the condenser. This start-up bypass system, which places the reheater and superheater in series, involves additional system complexity and cost.
As noted previously, one present practice is to utilize a single steam turbine and a single HRSG. This arrangement precludes the potential for mismatch between main steam flow and reheat flow, and also enables convenient start-up of the HRSG steam turbine. It is otherwise disadvantageous in that one steam turbine is required for each gas turbine in a combined cycle, again increasing plant cost.
For combined cycle systems, it is desirable, therefore, to configure the steam cycle with multiple gas turbines and HRSG's, and one steam turbine. It is therefore the principal object of this invention to provide a steam cycle arrangement which tolerates mismatch of main and reheat steam flow, i.e., any mismatch which does occur will not result in excessively high temperature in the reheater. In addition, the present invention seeks to eliminate the current requirement for bypassing main steam from the superheater through the reheater during HRSG start-up in multiple gas turbine/HRSG systems.
In accordance with an exemplary embodiment of the invention, the high temperature section of the superheater, and not the high temperature section of the reheater (as in prior designs), is the HRSG component that is exposed to the highest gas turbine exhaust temperature. The heat transfer duty of the superheater is sufficient to reduce the gas turbine exhaust temperature at the inlet to the reheater so that the potential for operation of the reheater above its rated operating temperature, and for damage as a result, is minimized. At the same time, the steam cycle arrangement enables start-up of the HRSG's without diverting superheater steam through the reheater. In the simplified system of this invention, superheater steam from the multiple HRSG's is diverted around the steam turbine to the condenser, while attemperation water is supplied to the reheater. This is sufficient to prevent overheating of the reheater during start-up because of the relocation of the high temperature section of the reheater behind the superheater, i.e., downstream in the direction of gas turbine exhaust flow.
Thus, in accordance with one aspect of the invention, there is provided a combined cycle power system in which condensate from a steam turbine is heated in at least one heat recovery steam generator by exhaust gas from at least one gas turbine, and wherein at least one heat recovery steam generator includes at least one superheater and at least one reheater, the improvement comprising locating at least a high temperature section of the superheater within the heat recovery steam generator so as to present first heat exchange surfaces to exhaust gas entering the heat recovery steam generator from said at least one gas turbine.
In accordance with another aspect of the invention, there is provided reheat steam cycle configuration for a steam turbine and gas turbine combined cycle system comprising: a steam turbine connected to a load; a condenser for receiving exhaust steam from the steam turbine and for condensing the exhaust steam to water; at least one heat recovery steam generator for receiving water from the condenser and for converting the water to steam for return to the steam turbine; at least one gas turbine for supplying heat to the heat recovery steam generator in the form of exhaust gases; wherein at least one heat recovery steam generator includes a reheater for receiving cold reheat steam from the steam turbine and a superheater for receiving high pressure steam from a high pressure evaporator within the heat recovery steam generator, and wherein the cold reheat steam and the high pressure steam flow in a direction opposite to that of the exhaust gases from the gas turbine, a high temperature section of the superheater being located within the heat recovery steam generator where gas turbine exhaust gas temperatures are highest.
In accordance with still another aspect of the invention, there is provided a method for supplying steam to a steam turbine from a plurality of heat recovery steam generators, each of which includes a plurality of evaporators, a reheater and a superheater and wherein at least the reheater and superheater are arranged in heat exchange relationship with hot exhaust gas from a gas turbine, the method comprising the steps of:
a) locating a high temperature section of the superheater adjacent a gas turbine exhaust gas inlet to the heat recovery steam generator; PA1 b) locating a high temperature section of the reheater adjacent the superheater, downstream of the high temperature section of the superheater in a direction of flow of the gas turbine exhaust gas; and PA1 c) flowing condensate and steam from the steam turbine in a direction opposite the direction of flow of the gas turbine exhaust gas so that the gas turbine exhaust gas temperature is reduced when it passes over the reheater to thereby minimize potential overheating of the reheater.
Other objects and advantages of the subject invention will become apparent from the detailed description which follows.